This invention relates to printing inks and more particularly to infrared absorptive printing inks suitable for use in jet drop printers of the type disclosed in Brady et al, U.S. Pat. No. 3,805,273. Such printers print on a high-speed moving web by selective charging and catching of drops generated at a frequency in the order of about 50 kHz by each of the streams flowing from 500 or more orifices. These orifices have a diameter which may be less than about 2 mils, which requires that the printing ink be free of anything but the very smallest particulate matter. Generally speaking the ink must be operable after filtering through a 3 micron filter and preferably should be able to undergo filtering through a 0.65 micron filter.
Further, for use in such a jet drop printer the ink must be electrically conductive, having a resistivity below about 1000 ohm cm and preferably below about 500 ohm cm. For good runnability through small orifices the ink should have a viscosity in the range between about 1 to 10 centipose at 25.degree. C. Over and above this the ink must be stable over a long period of time, compatable with the materials comprising the orifice plate and ink manifold, free of living organisms, and functional after printing. The required functional characteristics after printing are: sufficient light absorptivity at infrared wave lengths, smear resistance after printing, fast drying on paper, and waterproof when dry.
It will be appreciated that it is quite difficult to provide an ink which performs as required after printing and yet is suitable for use in such an ink jet printer. Generally speaking, water base inks have been found to be most suitable for use in such printers because of their conductivity and viscosity range, but heretofore there has been no such ink having fully satisfactory smudge-resisting, drying and waterproof characteristics. Moreover, the above problems, which exist for jet printing inks of all colors, are particularly severe when the ink is required to print characters which are readable by OCR readers operating in the infrared range of about 7,000-11,000 angstroms.
For many applications calling for infrared absorptive jet printing ink there is a concomitant requirement that the jet printed images be readable to the human eye. Generally speaking, human beings are accustomed to reading black printing, and it is therefore highly desirable that infrared absorptive jet printing inks be visually "black" as well as infrared absorbing. Accordingly prior art jet printing inks have avoided certain green dyes known to have infrared absorbing characteristics and generally have utilized water soluble nigrosine black dyes.
Typical prior art jet printing inks using nigrosine dye as an infrared absorbing agent are disclosed in Zabiak U.S. Pat. No. 3,705,043 and in Edds U.S. Pat. No. 3,870,528. While nigrosine is quite black to the eye and is a good infrared absorber, it tends to agglomerate and is difficult to maintain in suspension for a long time. As taught by Edds et al there are solvents which greatly reduce such agglomeration, and inks comprising such dye solvent compositions apparently perform satisfactorily in jet printers having relatively few jets. Such inks, however, have not been fully satisfactory in printers of the type disclosed in the above mentioned Brady et al patent.